1. Field of the Invention
The present invention relates to a body biasing device and an operational amplifier thereof, and more particularly, to a body biasing device capable of changing a body bias of a P-type differential pair of an input stage in an operational amplifier according to a node voltage related to differential input signals of the operational amplifier and an operational amplifier thereof.
2. Description of the Prior Art
Operational amplifiers are basic circuit elements in analog integrated circuitry. Generally speaking, an input stage of an operational amplifier in the prior art commonly uses P-type differential pair to acquire smaller noise, larger slew rate and higher unity-gain frequency.
For example, please refer to FIG. 1, which introduces a schematic diagram of an operational amplifier 10, wherein a supply voltage range of the operational amplifier 10 is within a voltage VCC to a voltage −VCC. In brief, an input stage 100, an output stage 102 and a current source CS of the operational amplifier 10 are shown in FIG. 1, circuitry unrelated to the concept of the present invention, such as a gain stage and a bias circuit, are not shown in FIG. 1. The input stage 100 comprises a P-type differential pair consisting of a PMOS P_IN and a PMOS P_IP, and is utilized for outputting a differential output signal VOUTN and a differential output signal VOUTP according to a differential input signal VIN and a differential input signal VIP.
The output stage 102 is utilized for receiving the differential output signal VOUTN and the differential output signal VOUTP and accordingly outputting an output signal VOUT at an output node OUT. The current source CS provides an operating current of the input stage 100. Besides, since a substrate of integrated circuitry normally is a P-substrate consisting of P-type semiconductor and a body of a PMOS is N-well built in the substrate and consisting of N-type semiconductor, a P-N junction is formed between the P-substrate and the N-well. In other words, the P-substrate and the N-well equal a diode. Therefore, a parasitic diode Dpar is shown in FIG. 1. The parasitic diode Dpar is between a ground GND and a body of the PMOS P_IN and a body of the PMOS P_IP, and is utilized for representing the non-ideal effects between the P-substrate and the N-well.
When swing of the differential pair signals VIN, VIP are too large, a forward bias Vdiode might be larger than a P-N junction forward conduction voltage Von, i.e. a source voltage VS of the PMOS P_IN, P_IP might be smaller than the negative P-N junction forward conduction voltage −Von (VS<−Von). As a result, the parasitic diode Dpar is conducted. In such a condition, an additional current will feed into the input stage 100 and result in the input stage 100 working abnormally.
In general, if the input stage of the operational amplifier is an N-type differential pair, i.e. the differential pair consisted of NMOS, the above mentioned additional current is not generated and the input stage can work normally. However, if the input stage of the operational amplifier 10 only uses the N-type differential pair, the operational amplifier 10 cannot achieve rail-to-rail structure and the input common mode range of the operational amplifier is smaller. In addition, if the input stage of the operational amplifier 10 is the N-type differential pair, the performance of the noise, slew rate and the unity-gain frequency of the operational amplifier degrade.
Another method for preventing the additional current feeding into the input stage 100 is using high voltage devices to achieve the operational amplifier 10 and coupling the body of the PMOS P_IN, P_IP to the highest voltage of the operational amplifier 10 (i.e. voltage VCC). However, using the high voltage devices increases cost of the integrated circuitry. In addition, the performance of the high voltage devices is poorer and the noise of the high voltage devices is larger, such that the performance of the operational amplifier 10 degrades. Thus, there is a need to improve the prior art.